SiAlON ceramics and a method of preparation thereof

ABSTRACT

A Ca—SiAlON ceramic with enhanced mechanical properties and a method employing micron-sized and submicron precursors to form the Ca—SiAlON ceramic. The Ca—SiAlON ceramic comprises not more than 42 wt % silicon, relative to the total weight of the Ca—SiAlON ceramic. The method employs submicron particles and also allows for substituting a portion of aluminum nitride with aluminum to form the Ca—SiAlON ceramic with enhanced mechanical properties.

STATEMENT OF ACKNOWLEDGEMENT

The authors wish to acknowledge King Abdul-Aziz City for Science andTechnology (KACST) represented by the science and technology unit inKing Fand University for Petroleum and Minerals (KFUPM) for funding thiswork through the National Science, Technology and Innovation Plan(NSTIP) with a project no. 12-ADV2411-04.

BACKGROUND

Field of the Disclosure

The present disclosure relates to SiAlON ceramics with enhancedmechanical properties and a method of making these SiAlON ceramics.

Discussion of the Background

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Silicon nitride (Si₃N₄) ceramics have been shown to withstand severeworking conditions due to their remarkable mechanical and thermalproperties, namely hot hardness, chemical inertness and thermal shockresistance (Riley, Frank L. Journal of the American Ceramic Society 83.2(2000): 245-265, incorporated herein by reference in its entirety).However, full densification of Si₃N₄ powder under the pressure andtemperature provided by conventional solid-state sintering techniqueshas been challenging. The difficulty is attributed to the strongcovalent Si—N bond, which resists plastic deformation (Liu, Limeng, etal. Journal of the European Ceramic Society 30.12 (2010): 2683-2689;Belmonte, M., et al. Journal of the European Ceramic Society 30.14(2010): 2937-2946, each incorporated herein by reference in theirentirety). Therefore, Jack and Oyama have developed SiAlON ceramics thatare solid solutions of alumina oxide (Al₂O₃) in Si₃N₄, in which aportion of silicon and nitrogen is replaced by aluminum and oxygen,respectively (Jack, K. H., and W. I. Wilson. Nature Physical Science(1972): 28-29; Oyama, Yoichi, and Osami Kamigaito. Japanese Journal ofApplied Physics 10.11 (1971): 1637, Jack, K. H. Journal of materialsscience 11.6 (1976): 1135-1158, each incorporated herein by reference intheir entirety).

To prepare a fully dense ceramic, Al₂O₃ is mixed with Si₃N₄ in thesintering process to form a pure SiAlON ceramic (Cao, G. Z., and RuudMetselaar. Chemistry of Materials 3.2 (1991): 242-252, incorporatedherein by reference in their entirety). Based on the composition of theinitial powder mixture and the sintering parameters, several phases,including α-SiAlON, β-SiAlON, O—SiAlON, X—SiAlON or a mixture of them,can form and exist in the SiAlON ceramic (Zhou, Y., et al. Journal ofMaterials science 30.18 (1995): 4584-4590; Vleugels, Jozef, et al.Institute of physics conference series. No. 130. 1993; Laoui, Tahar, andOmer Van der Biest. Key Engineering Materials. Vol. 89. 1993, eachincorporated herein by reference in their entirety). In particular,α-SiAlON and β-SiAlON have attracted attention in the past two decadesdue to their favorable mechanical properties: high hardness for α-SiAlONand reasonable fracture toughness for β-SiAlON (Izhevskiy, V. A., et al.Journal of the European Ceramic Society 20.13 (2000): 2275-2295;Ekström, Thommy, and Mats Nygren. Journal of the American CeramicSociety 75.2 (1992): 259-276, each incorporated herein by reference intheir entirety).

Later, researchers explored other sintering aids to improve the ceramicdensification at a lower energy input (i.e. lower sintering temperatureand a shorter sintering time) and to produce ceramics with optimalmechanical and optical properties. Lanthanides, such as Nd, La and Yb,have been studied (Herrmann, Mathias, Sören Höhn, and Axel Bales.Journal of the European Ceramic Society 32.7 (2012): 1313-1319; Menke,Yvonne, Valerie Peltier-Baron, and Stuart Hampshire. Journal ofnon-crystalline solids 276.1 (2000): 145-150; Bandyopadhyay, Siddhartha,M. J. Hoffmann, and G. Petzow. Ceramics international 25.3 (1999):207-213; Hakeem, Abbas Saeed, Jekabs Grins, and Saeid Esmaeilzadeh.Journal of the European Ceramic Society 27.16 (2007): 4773-4781, eachincorporated herein by reference in their entirety). However, thelanthanides have large ionic radii and they poorly occupy theinterstitial sites of Si₃N₄, producing ceramics with crystal defects. Inaddition, introducing lanthanides into ceramics is alsocost-prohibitive.

In view of the foregoing, the objective of the present disclosure is toprovide SiAlON ceramics with enhanced mechanical properties and a methodof producing these SiAlON ceramics.

SUMMARY OF THE DISCLOSURE

The foregoing description is intended to provide a general introductionand summary of the present disclosure and is not intended to be limitingin its disclosure unless otherwise explicitly stated. The presentlypreferred embodiments, together with further advantages, will be bestunderstood by reference to the following detailed description taken inconjunction with the accompanying drawings.

A first aspect of the disclosure relates to a process for producing aCa—SiAlON ceramic, comprising: (i) mixing (a) calcium oxide, (b) siliconnitride, (c) alumina, (d) aluminum nitride, and (e) aluminum to form apowder mixture, (ii) sintering the powder mixture to form the Ca—SiAlONceramic, and (iii) cooling the Ca—SiAlON ceramic to a temperatureranging from 20-40° C.

In one embodiment, the sintering is performed in an atmosphereconsisting essentially of nitrogen gas.

In one embodiment, the mixing is at least one of sonication and ballmilling.

In one embodiment, the mixing proceeds for 10-30 minutes.

In one embodiment, the calcium oxide is in the form of particles with adiameter ranging from 1-200 nm.

In one embodiment, an amount of the silicon nitride ranges from 40 wt %to less than 80 wt % relative to a total weight of the powder mixture.

In one embodiment, the silicon nitride is α-Si₃N₄ and in the form ofparticles with a diameter ranging from 1-200 nm.

In one embodiment, the silicon nitride is amorphous and in the form ofparticles with a diameter ranging from 1-100 nm.

In one embodiment, the alumina is in the form of particles with adiameter ranging from 1-200 nm.

In one embodiment, the aluminum nitride is in the form of particles witha diameter ranging from 1-200 nm.

In one embodiment, the aluminum is in the form of particles with adiameter ranging from 10-100 μm.

In one embodiment, an amount of the aluminum ranges from more than 0 wt% to 10 wt % relative to a total weight of the powder mixture.

In one embodiment, the sintering is a spark plasma sintering process.

In one embodiment, the sintering is performed at a temperature rangingfrom 1400-1600° C.

In one embodiment, the sintering comprises heating the powder mixture ata rate ranging from 5-600° C./min.

In another embodiment, the rate ranges from 90-110° C./min.

In one embodiment, the sintering comprises applying a uniaxial pressureranging from 20-150 MPa to the powder mixture.

In another embodiment, the uniaxial pressure ranges from 45-55 MPa.

A second aspect of the disclosure relates to a Ca—SiAlON ceramic,comprising: (i) calcium, (ii) silicon in an amount less than 42 wt % ofa total weight of the Ca—SiAlON ceramic, (iii) aluminum, (iv) oxygen,and (v) nitrogen, where the Ca—SiAlON ceramic comprises at least one ofthe following phases: a β-SiAlON phase, an α-SiAlON phase, silicondioxide, a SiAl_(y)O₂N_(y) phase, wherein y ranges from 4 to 6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows the XRD pattern of sample 3-α (sintered at 1600° C.),which comprises β-SiAlON (▪) and Si_(3.1)Al_(2.9)O_(2.9)N_(5.1) (▴).

FIG. 1B shows the XRD pattern of sample 3-α (0.1Al) (with 10 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (●), Si_(3.1)Al_(2.9)O_(2.9)N_(5.1) (▴) andSiAl₅O₂N₅ (◯).

FIG. 1C shows the XRD pattern of sample 3-α-(0.2Al) (with 20 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (●), SiAl₂O₂N₅ (◯), SiAl₆O₂N₆ (Δ) and SiO₂ (⊕).

FIG. 1D shows the XRD pattern of sample 3-α(0.3Al) (with 30 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (●), SiAl₅O₂N₅ (◯), SiAl₆O₂N₆ (Δ) and SiO₂ (⊕).

FIG. 2A shows the XRD pattern of sample 3-Amp (sintered at 1600° C.),which comprises β-SiAlON (▪) and SiAl₆O₂N₆ (Δ).

FIG. 2B shows the XRD pattern of sample 3-Amp(0.1Al) (with 10 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises β-SiAlON (▪) and SiAl₆O₂N₆ (Δ).

FIG. 2C shows the XRD pattern of sample 3-Amp(0.2Al) (with 20 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises β-SiAlON (▪) and SiAl₄O₂N₄ (+).

FIG. 2D shows the XRD pattern of sample 3-Amp(0.3Al) (with 30 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises β-SiAlON (▪) and SiAl₄O₂N₄ (★).

FIG. 3 is a SEM micrograph of sample 3-α (sintered at 1600° C.), whichcomprises β-SiAlON (β) and Si_(3.1)Al_(2.9)O_(2.9)N₅ (▴).

FIG. 4 is a SEM micrograph of sample 3-α (sintered at 1600° C.), whichcomprises β-SiAlON (β) and Si_(3.1)Al_(2.9)O_(2.9)N₅ (▴).

FIG. 5 is a SEM micrograph of sample 3-α(0.1Al) (with 10 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (α), Si_(3.1)Al_(2.9)O_(2.9)N_(5.1) (▴) andSiAl₅O₂N₅ (→).

FIG. 6 is a SEM micrograph of sample 3-α(0.2Al) (with 20 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (α), SiAl₆O₂N₆ (→) and SiO₂ (×).

FIG. 7 is a SEM micrograph of sample 3-α(0.3Al) (with 30 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (α) and SiAl₆O₂N₆ (→).

FIG. 8 is a SEM micrograph of sample 3-Amp (sintered at 1600° C.), whichcomprises β-SiAlON (β) and SiAl₆O₂N₆ (→).

FIG. 9 is a SEM micrograph of sample 3-Amp(0.1Al) (with 10 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (α), β-SiAlON (β) and SiAl₅O₂N₅ (→).

FIG. 10 is a SEM micrograph of sample 3-Amp(0.1Al) (with 10 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises α-SiAlON (α) and SiAl₅O₂N₅ (→).

FIG. 11 is a SEM micrograph of sample 3-Amp(0.2Al) (with 20 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises β-SiAlON (β) and SiAl₄O₂N₄ (

).

FIG. 12 is a SEM micrograph of sample 3-Amp(0.3Al) (with 30 mol % ofaluminum nitride substituted with aluminum and sintered at 1600° C.),which comprises β-SiAlON (β) and SiAl₄O₂N₄ (

).

FIG. 13 is a SEM micrograph showing the elongated α-SiAlON grains insample 3-α(0.3Al) (with 30 mol % of aluminum nitride substituted withaluminum and sintered at 1600° C.).

FIG. 14 is a SEM micrograph of sample 3-α (sintered at 1450° C.).

FIG. 15 is a SEM micrograph of sample 3-α(0.1Al) (with 10 mol % ofaluminum nitride substituted with aluminum and sintered at 1450° C.).

FIG. 16 is a SEM micrograph of sample 3-α(0.2Al) (with 20 mol % ofaluminum nitride substituted with aluminum and sintered at 1450° C.).

FIG. 17 is a SEM micrograph of sample 3-α(0.3Al) (with 30 mol % ofaluminum nitride substituted with aluminum and sintered at 1450° C.).

DETAILED DESCRIPTION OF THE DISCLOSURE

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

Within the description of this disclosure, where a numerical limit orrange is stated, the endpoints are included unless stated otherwise.Also, all values and subranges within a numerical limit or range arespecifically included as if explicitly written out. As used herein, thewords “a” and “an” and the like carry the meaning of “one or more”.

Improving ceramic densification at a lower energy input (i.e. lowersintering temperature and a shorter sintering time) to produce ceramicswith favorable mechanical properties has been a challenge for theresearch community. One approach is to employ sintering aids such aslanthanides, alkaline earth metals and/or oxides of alkaline earthmetals. Among these candidates, calcium oxide seems the most promisingbecause the calcium atom can reside in α-SiAlON structure withoutdistorting the crystal structure (Wang, P. L., et al. Materials Letters38.3 (1999): 178-185; Wang, P. L., Y. W. Li, and D. S. Yan. Journal ofthe European Ceramic Society 20.9 (2000): 1333-1337, each incorporatedherein by reference in their entirety). In addition, calcium compoundsare more economical than lanthanide compounds (Van Rutten, J. W. T., H.T. Hintzen, and Ruud Metselaar. Ceramics international 27.4 (2001):461-466, incorporated herein by reference in its entirety). Anotherapproach to achieve ceramic densification at a lower sinteringtemperature is to employ submicron-sized and/or nano-sized startingpowders, as described hereinafter.

Therefore, the first aspect of this disclosure relates to a process forproducing a Ca—SiAlON ceramic, the process comprising: (i) mixing (a)calcium oxide, (b) silicon nitride, (c) alumina, (d) aluminum nitride,and (e) aluminum to form a powder mixture, (ii) sintering the powdermixture to form the Ca—SiAlON ceramic, and (iii) rapidly cooling theCa—SiAlON ceramic to about 20-40° C., where at least one of theaforementioned components are submicron-sized or nano-sized. In oneembodiment, the process is performed in an atmosphere consistingessentially of nitrogen gas. In a preferred embodiment, the atmosphereis nitrogen gas with a purity of more than 99.99%.

The aforementioned components may be clusters, aggregates, powders orparticles. In a preferred embodiment, the components are particles. Theaforementioned components may be mixed by sonication and/or ballmilling. In a preferred embodiment, the powders are dispersed in asufficient amount of an organic solvent, preferably volatile at roomtemperature, to form a slurry and sonicated for 10-30 minutes,preferably 15-25 minutes, with an ultrasonic probe. Non-limitingexamples of the organic solvent include hydrocarbons, such as hexane,alcohols, such as ethanol, methanol, propanol, isopropanol, butanol,ketones and esters. Preferably, the solvent is an alcohol. Morepreferably, the alcohol has a melting point lower than 0° C. and aboiling point lower than 100° C. In a preferred embodiment, the alcoholis ethanol. The organic solvent may act as a viscosity modifying agent,providing a suitable viscosity for handling the slurry and accomplishingthe mixing. In addition, the solvent may have a viscosity ranging from0.5-2 cP, preferably 0.5-1.5 cP, more preferably 0.5-1.2 cP. Any amountof liquid that accomplishes the mixing is acceptable. Preferably, thesolids content is between 15-50 vol %, preferably 15-35 vol %, morepreferably 20-30 vol % of the total volume of the slurry. Below thislimit mixing may be ineffective or separation by settling may occur,although a solid content below this limit may still be used depending onthe particle size, solvent, and mixing procedure. Above the limit, insome instances, the viscosity may be too high and mixing andde-agglomeration may not be effective. The volatile organic solvent mayevaporate during sonication, leaving no residue. Preferably, the slurryis heated to 75-85° C. for 2-24 hours, preferably 10-24 hours, morepreferably 16-24 hours to remove the solvent completely.

In one embodiment, the powders are milled with a miller, such as aplanetary miller, an attrition mill, a vibratory mill or a high energymiller. Non-limiting examples of milling media (i.e. bowl and balls)include tungsten carbide, silicon nitride and alumina. Preferably,silicon nitride milling media is employed to minimize contamination ofthe powder mixture. In one embodiment, a weight ratio of the balls tothe powder mixture ranges from 5:1 to 20:1, preferably from 5:1 to 10:1,more preferably from 7:1 to 10:1. A process control agent, such asstearic acid, may be added to the powder mixture to ensure the powdermixture does not cake. An amount of the process control agent rangesfrom more than 0 wt % to 2 wt %, preferably 0.5-1.5 wt %, morepreferably 0.5-1 wt % of the weight of the powder mixture. In apreferred embodiment, no process control agent is employed. In oneembodiment, the milling is performed in an inert atmosphere, preferablyprovided by argon gas. The powder mixture may be milled for up to 10hours, or up to 5 hours, or up to 1 hour, preferably for 10-30 minutes,preferably for 15-25 minutes.

The calcium oxide particles, silicon nitride particles, aluminaparticles, aluminum nitride particles and aluminum particles may bespheres, spheroids, ellipsoids, flakes or irregular shapes, unlessotherwise specified. In a preferred embodiment, the particles arespheres or substantially spherical. A diameter of the particle, as usedherein, refers to the greatest possible distance measured from one pointon the particle through the center of the particle to a point directlyacross from it. A diameter of a flake, as used herein, refers to thegreatest possible distance measured from a first point on a perimeter ofthe flake through the center of the flake to a second point, also on theperimeter of the flake, directly across from the first point. Thediameters of the particles are described hereinafter.

The calcium oxide particles have a diameter ranging from 1-200 nm,preferably 10-180 nm, more preferably 80-160 nm. A purity of calciumoxide particles is more than 95 wt %, preferably more than 97 wt %, andmore preferably more than 98 wt % relative to the total weight of thecalcium oxide particles. An amount of the calcium oxide particles in thepowder mixture ranges from more than 0 wt % to 20 wt %, preferably 3-10wt %, more preferably 8-10 wt % of the total weight of the powdermixture.

An amount of the silicon nitride particles in the powder mixture rangesfrom 40 wt % to less than 80 wt %, preferably 40-60 wt %, morepreferably 40-50 wt % relative to the total weight of the powdermixture. A purity of silicon nitride particles is more than 98 wt %,preferably more than 99 wt %, and more preferably more than 99.9 wt %relative to the total weight of the silicon nitride particles. Thesilicon nitride particles may be α-Si₃N₄, β-Si₃N₄, γ-Si₃N₄ and amorphousSi₃N₄ and mixtures thereof. In a preferred embodiment, the siliconnitride particles are α-Si₃N₄ and have a diameter ranging from 1-200 nm,preferably 10-180 nm, more preferably 80-150 nm. In another embodiment,the silicon nitride particles are amorphous and have a diameter rangingfrom 1-100 nm, preferably 10-50 nm, more preferably 10-30 nm.

The alumina particles have a diameter ranging from 1-200 nm, preferably10-180 nm, more preferably 80-150 nm. The alumina may be α-alumina,γ-alumina, η-alumina, θ-alumina, δ-alumina, χ-alumina and κ-alumina. Ina preferred embodiment, α-alumina is employed. A purity of the aluminaparticles is at least 99 wt %, preferably at least 99.5 wt %, morepreferably at least 99.8 wt % relative to the total weight of thealumina particles. An amount of the alumina particles in the powdermixture ranges from more than 0 wt % to 20 wt %, preferably 5-18 wt %,more preferably 15-18 wt % of the total weight of the powder mixture.

The aluminum nitride particles have a diameter ranging from 1-200 nm,preferably 10-180 nm, more preferably 80-150 nm. A purity of aluminumnitride particles is more than 98 wt %, preferably more than 99 wt %,and more preferably more than 99.9 wt % relative to the total weight ofthe aluminum nitride particles. An amount of the aluminum nitrideparticles in the powder mixture ranges from more than 0 wt % to 30 wt %,preferably 10-30 wt %, more preferably 20-30 wt % of the total weight ofthe powder mixture.

Adding aluminum metal to the powder mixture may increase the amount of aliquid phase in the ceramic during sintering and thus may lead toceramic densification and a ceramic with enhanced mechanical properties.A portion of aluminum nitride particles may be replaced by aluminumparticles, and this portion ranges from 5-50 mol %, preferably 5-40 mol%, more preferably 10-30 mol % of the original amount of aluminumnitride particles in a ceramic sample prepared without aluminumparticles. The lesser amount of nitrogen when aluminum particles areemployed may be compensated by sintering the powder mixture in thepresence of nitrogen gas, which reacts with aluminum particles to formaluminum nitride under the sintering conditions.

An amount of aluminum particles, expressed in wt % of the total weightof the powder mixture, ranges from more than 0 wt % to 10 wt %,preferably 0.8-8 wt %, more preferably 1-6 wt %. The aluminum particleshave a diameter ranging from 10-100 μm, preferably 30-70 μm, morepreferably 40-50 μm. A purity of aluminum particles is more than 95 wt%, preferably more than 99 wt %, more preferably more than 99.95 wt %relative to the total weight of the aluminum particles.

The sintering process may be hot pressing, hot isostatic pressure,pressureless sintering or spark plasma sintering. Preferably, thesintering is a spark plasma sintering process because this processdensifies the compacted powders faster than the aforementioned processesand at relatively low temperatures (Salamon, David, Zhijian Shen, andPavol {hacek over (S)}ajgalik. Journal of the European Ceramic Society27.6 (2007): 2541-2547, incorporated herein by reference in itsentirety). Thus, the formation of unfavorable secondary phases withpoorer mechanical properties may be minimized. A ratio of the weight ofthe secondary phases relative to the weight of the primary phases (e.g.a β-SiAlON phase, an α-SiAlON phase, silicon dioxide and aSiAl_(y)O₂N_(y) phase) may range from 1:199 to 5:95, preferably 1:199 to5:145, more preferably 1:199 to 1:99. The ratio may be estimated bycomparing the peak area ratio of the XRD patterns.

The powder mixture may be poured into a graphite die with a diameter of10-50 mm, preferably 15-35 mm, more preferably 15-25 mm. The powdermixture may be compacted at ambient temperature. In a preferredembodiment, a uniaxial pressure is applied to the die in a directionthat is normal to the ground. The pressure ranges from 20-150 MPa,preferably 20-100 MPa, more preferably 45-55 MPa. The sintering involvesheating the powder mixture at a rate ranging from 5-600° C./min,preferably 50-200° C./min, more preferably 90-110° C./min. The heatingmay comprise of at least one heating step. In a preferred embodiment,the heating consists of only one heating step. The sintering isperformed at a temperature ranging from 1400-1900° C., preferably1400-1700° C., more preferably 1400-1600° C. The Ca—SiAlON ceramicstarts to cool down once the current is switched off. The cooling of theCa—SiAlON ceramic may be controlled and/or accelerated with a pre-setprogram. In a preferred embodiment, the ceramic is cooled down at a rateranging from 1-200° C./s, preferably 1-100° C./s, more preferably 5-100°C./s by a flow of nitrogen gas. The ceramic may be cooled to atemperature ranging from 20-40° C., preferably 20-30° C., morepreferably 20-25° C.

The second aspect of this disclosure relates to a Ca—SiAlON ceramic,comprising: (i) calcium, (ii) silicon, (iii) aluminum, (iv) oxygen, and(v) nitrogen, where the Ca—SiAlON ceramic comprises at least of thefollowing phases: a β-SiAlON phase, an α-SiAlON phase, silicon dioxide,a SiAl_(y)O₂N_(y) phase, wherein y ranges from 4-6, preferably 5-6, morepreferably about 6. In one embodiment, y is a whole number and not aninteger. In a preferred embodiment, y is an integer. A N:O ratio of thealuminum silicon oxynitride phases (e.g. the β-SiAlON phase and theSiAl_(y)O₂N_(y) phase) ranges from 5:3 to 3:1, preferably 5:2 to 3:1,more preferably about 3:1. The presence of these crystalline phases maybe measured by methods such as XRD analysis. In one embodiment, theCa—SiAlON ceramic consists of: (i) calcium, (ii) silicon, (iii)aluminum, (iv) oxygen, and (v) nitrogen.

The α-SiAlON phase may be of the formulaM_(x)Si_(12−(m+n))Al_(m+n)O_(n)N_(16−n), where M represents a lanthanidemetal cation, such as cerium, gadolinium and promethium, etc. and/or analkaline earth metal cation, such as calcium, barium and magnesium, x isa whole number which ranges from more than 0 to 2, m is a whole numberwhich ranges between 0.9-3.5, n is a whole number which ranges from morethan 0 to 2. In a preferred embodiment, calcium atoms are present in theα-SiAlON phase (i.e. M is Ca) and the ceramic may be of the formulaCaSi₆Al₆O₄N₁₂ (x=1, m=2, n=4).

The β-SiAlON phase is of the formula Si_(6−z)Al_(z)O_(z)N_(8−z) where zranges from more than 0 to less than 4.2 (Jack, K. H. Journal ofMaterials Science 11 (1976):1135-1158, incorporated herein by referencein its entirety). In a preferred embodiment, z ranges from 1-4, morepreferably 2-4.

Weight percentages (relative to the total weight of the Ca—SiAlONceramic) of the calcium, silicon, aluminum, oxygen and nitrogen in theCa—SiAlON ceramic are described hereinafter. An amount of calcium rangesfrom more than 0 wt % to 7.5 wt %, preferably 2-7.5 wt %, morepreferably 5-7.5 wt %. The amount of silicon is less than 42 wt %,preferably ranges from 24 wt % to less than 42 wt %, more preferably24-36 wt %, and even more preferably 24-30 wt %. An amount of aluminumis at most 30.4 wt %, preferably ranges from 9.2-29.4 wt %, morepreferably 21.2-29.4 wt %. An amount of oxygen is at most 12.5 wt %,preferably 3-11.5 wt %, more preferably 9-11.5 wt %. An amount ofnitrogen ranges from 16-38.2 wt %, preferably 19.4-34.2 wt %, preferably22.8-30.2 wt %. The aforementioned weight percentages of calcium,silicon, aluminum, oxygen and nitrogen may be measured by atomicabsorption spectroscopy, the energy dispersive X-ray technique, thecarrier-gas-heat-extraction method and a combination thereof (Beck, H.P. et al. Fresenius' Journal of Analytical Chemistry, 357.6(1999):652-655, incorporated herein by reference in its entirety).

The grain size and morphology of the Ca—SiAlON ceramic are studied withelectron microscopy, preferably scanning electron microscopy (SEM). TheCa—SiAlON ceramic has micropores and submicron pores. The size of themicropores ranges from 1-5 μm, preferably 1-4 μm, more preferably 1-3μm. The size of the submicron pores ranges from 50-400 nm, preferably100-300 nm, more preferably 100-250 nm. The porosity of the Ca—SiAlONceramic is at most 20%, preferably at most 15%, preferably at most 5%,and more preferably at most 1%.

The grains of the Ca—SiAlON ceramic may be elongated, equiaxed, needlesand/or flakes. The elongated grains may have a height ranging from 1-20μm, preferably 1-15 μm, more preferably 1-10 μm, and a width rangingfrom 0.1-4 μm, preferably 0.5-3 μm, more preferably 0.5-2 μm. The aspectratio of the elongated grains ranges from 2-200, preferably 10-150, morepreferably 10-100. The elongated grains may be 3-SiAlON grains. Theequiaxed grains have a diameter ranging from 100-1,000 nm, preferably200-900 nm, more preferably 300-500 nm. The equiaxed grains may beα-SiAlON grains. The needle-like grains have a length ranging from 1-20μm, preferably 1-15 μm, more preferably 1-10 μm. The flake-like grainshave a diameter of 1-15 μm, preferably 5-15 μm, more preferably 7-10 μm.The needle-like and flake-like grains may be β-SiAlON grains.

The properties of the SiAlON material are readily measured by standardtests. In particular, for the purposes of this disclosure, the fracturetoughness of the ceramic is evaluated by the indentation technique, andthe hardness with the Vickers hardness method (10 kg load) employing auniversal hardness tester.

Substituting a portion of aluminum nitride particles with aluminum hasled to the formation Ca—SiAlON ceramics with enhanced mechanicalproperties. In most embodiments, the Ca—SiAlON ceramic has a Vickershardness ranging from 11.5-24.9 GPa, preferably 13.1-19.9 GPa, morepreferably 15.5-19.9 GPa and a fracture toughness ranging from 3.9-11.4MPa√m, preferably 5.7-11.4 MPa√m, more preferably 7-11.4 MPa√m. In oneembodiment, where 15-35 mol % of aluminum nitride is substituted byaluminum and the powder mixture is sintered at 1600° C., the Vickershardness of the ceramic ranges from 15.5-24.9 GPa and the fracturetoughness ranges from 7.6-11.4 MPa√m. In one embodiment, where about 20mol % of aluminum nitride is substituted by aluminum and the powdermixture is sintered at about 1600° C., the Vickers hardness of theceramic is about 16 GPa and a fracture toughness of about 9.5 MPa√m. Inanother embodiment, where about 30 mol % of aluminum nitride issubstituted by aluminum and the powder mixture is sintered at about1600° C., the Vickers hardness of the ceramic is about 18.5 GPa and afracture toughness of about 8.3 MPa√m.

In another embodiment, where 5-35 mol % of aluminum nitride issubstituted with aluminum and the powder mixture is sintered at 1450°C., the Vickers hardness of the ceramic ranges from 18.2-19.9 GPa andthe fracture toughness ranges from 5.7-8.6 MPa√m. In one embodiment,where about 10 mol % of aluminum nitride is substituted with aluminumand the powder mixture sintered at about 1450° C., the Vickers hardnessof the ceramic is about 18.6 GPa and a fracture toughness of about 7.8MPa√m. In another embodiment, where about 20 mol % of aluminum nitrideis substituted with aluminum and the powder mixture sintered at about1450° C., the Vickers hardness of the ceramic is about 19.1 GPa and afracture toughness of about 6.4 MPa√m. In one embodiment, where about 30mol % of aluminum nitride is substituted by aluminum and the powdermixture sintered at about 1450° C., the Vickers hardness is about 19.6GPa and a fracture toughness of about 6.5 MPa√m.

The improvement in fracture toughness may be due to a crack deflectionmechanism inherently supported by the mixed-morphology microstructure ofthe ceramic. The improvement in the Vickers hardness corresponded withan increase in a proportion of the α-SiAlON phase in the ceramic. Ingeneral, the α-SiAlON phase is harder than the β-SiAlON phase, which ismore resistant to fracture. The reason behind this variation in themechanical properties is explained through the consideration of thephase morphology, where the α-SiAlON grains are mostly equiaxed, whilethe β-SiAlON grains tend to be elongated.

The mechanical properties of the Ca—SiAlON ceramic may be altered byvarying the weight ratio of the α-SiAlON and the β-SiAlON phases. Asmentioned above, increasing the proportion of the α-SiAlON phaseincreases the hardness of the ceramic. In practice, varying the weightratio may be accomplished by substituting a portion of aluminum nitrideparticles with aluminum particles. For instance, a portion of aluminumnitride particles may be replaced by aluminum particles, and thisportion ranges from 5-50 mol %, preferably 5-40 mol %, more preferably10-30 mol % of the original amount of aluminum nitride particles in aceramic sample prepared without aluminum particles. The weight ratio ofthe α-SiAlON and the β-SiAlON phases may be measured by a peak arearatio of the XRD patterns, and the weight ratio ranges from 1:99 to99:1. Preferably, the weight ratio is between about 50:50 and 99:1. Morepreferably, the weight ratio is between 80:20 and 99:1, and morepreferably about 99:1.

Having generally described this disclosure, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

Example 1 Experimental Procedure

The starting materials were α-silicon nitride (150 nm—SN-10, Japan),amorphous silicon nitride (20 nm—Chempur, Germany), alumina (150nm—Chempur, Germany), aluminum nitride (100 nm—Sigma Aldrich, USA),calcium oxide (160 nm—Sigma Aldrich, USA) and aluminum metal (44 μm—LobaChemie, India).

For illustration purposes, the chemical composition of the ceramic wasfixed at CaSi₆Al₆O₄N₁₂, which corresponds to m=2 and n=4 in the generalformula of α-SiAlON. The chemical reaction formula for a sample withoutaluminum replacement can be written as follows:CaO+Al₂O₃+2Si₃N₄+4AlN→CaSi₆Al₆O₄N₁₂

To illustrate the substitution of aluminum nitride with aluminum, anexample of 10 mol % replacement is illustrated in the equation below. If10 mol % of aluminum nitride is substituted with aluminum, the equationmay be expressed as:CaO+Al₂O₃+2Si₃N₄+3.6AlN+0.4Al+0.2N₂→CaSi₆Al₆O₄N₁₂

To calculate the amount of aluminum metal to add to the powder mixture,one should multiply each reactant by its molar weight, as shown in thefirst row in Table 1. To get the required mass of aluminum for a sampleof 5 g, one should perform the following arithmetic operation(10.8/602.6)×5, which results in 0.0896 grams and corresponds to 1.8 wt% of the total weight of the powder mixture.

TABLE 1 Calculation example of 10% metallic aluminum replacement. Weightof powders (g) CaO Al₂O₃ Si₃N₄ AlN Al N Total (g) 56.1 102 280.6 147.610.8 5.6 602.6  0.4653   0.8461   2.3281   1.2244  0.0896 0.0465  5.0000

Table 2 lists the initial compositions for the powder mixtures withvarious amounts of aluminum.

TABLE 2 Weight percentages of the respective particles Sample CaO Al₂O₃α-Si₃N₄ Amp-Si₃N₄ AlN Al N 3-α 9.31 16.92 46.56 — 27.21 — — 3-α(0.1Al)9.31 16.92 46.56 — 24.49 1.79 0.93 3-α(0.2Al) 9.31 16.92 46.56 — 21.773.58 1.86 3-α(0.3Al) 9.31 16.92 46.56 — 19.05 5.37 2.79 3-Amp 9.31 16.92— 46.56 27.21 — — 3-Amp(0.1Al) 9.31 16.92 — 46.56 24.49 1.79 0.933-Amp(0.2Al) 9.31 16.92 — 46.56 21.77 3.58 1.86 3-Amp(0.3Al) 9.31 16.92— 46.56 19.05 5.37 2.79

5 g samples were weighed and mixed in ethanol with an ultrasonic probesonicator for 20 min. Samples were dried at 80° C. for 12 h to removeethanol.

The powder mixtures were poured into 20 mm diameter graphite dies. Sparkplasma sintering was performed at 1450° C. and 1600° C. for 30 min inthe presence of nitrogen, with an uniaxial pressing at 16 KN,corresponding to a pressure of about 50 MPa. A heating rate of 100°C./min was adopted to avoid formation of intermediate phases. To freezethe formed structure, samples were then rapidly cooled down to roomtemperature.

For characterization, samples were mounted, ground and polished inaccordance to the standard procedures. Rigaku MiniFlex X-raydiffractometer (Japan) was used to identify the phases present in thesynthesized samples, in which K_(Cu,α1)=0.15416 nm, accelerationvoltage=30 kV and tube current=10 mA. Samples were examined with a fieldemission scanning electron microscope (FESEM, Lyra 3, Tescan, CzechRepublic) using both SE and BSE detectors, with an acceleration voltageof 20-30 kV to characterize their morphologies, and energy dispersiveX-ray spectroscopy (EDX, Oxford Inc., UK) for elemental analysis of thepresent phases. Density was measured with the Archimedes method. Vickershardness (10 Kg load) was evaluated using the universal hardness tester(Zwick-Roell, ZHU250, and Germany). Fracture toughness was calculatedusing the well-known indentation method, utilizing Evan's Formula, whereMCL and d stand for maximum crack length and the impression diagonal,respectively (Hewett, C. L., Cheng, Y.-B., Muddle, B. C. and Trigg, M.B. J. Amer. Ceram. Soc. 81 (1998): 1781-1788, incorporated herein byreference in its entirety).

$K_{IC} = {0.48( \frac{MCL}{d/2} )^{- 1.5}( \frac{{HV}_{10}\sqrt{\frac{d}{2}}}{3} )}$

Example 2 Phases Present in Formed Ca—SiAlON Ceramics

FIG. 1 shows the XRD patterns of samples 3-α (FIG. 1A), 3-α(0.1Al) (FIG.1B), 3-α(0.2Al) (FIG. 1C) and 3-α(0.3Al) (FIG. 1D). There was a largequantity of β-SiAlON in sample 3-α. However, as more aluminum metal wasincorporated in the starting mixture, the amount of α-SiAlON increased,and the proportion of α-SiAlON was the largest in sample 3-α(0.3Al).Different aluminum silicon oxynitride phases were observed in differentproportions across the samples. The more aluminum metal is added to theceramic, the higher the nitrogen to oxygen ratio of the aluminum siliconoxynitride phase. For instance, in the Si_(3.1)Al_(2.9)O_(2.9)N_(5.1)phase in sample 3-α, has a N:O ratio of 5:3. In sample 3-α(0.1Al), theN:O ratio increased to 5:2 due to the presence of SiAl₅O₂N₅ phase.Further, an additional increase arose from the presence of SiAl₆O₂N₆phase in samples 3-α(0.2Al) and 3-α(0.3Al). A small amount of SiO₂ wasdetected in samples 3-α(0.2Al) and 3-α(0.3Al), with a slightly higherconcentration in the latter.

FIG. 2 presents the XRD patterns of samples 3-Amp (FIG. 2A),3-Amp(0.1Al) (FIG. 2B), 3-Amp(0.2Al) (FIG. 2C) and 3-Amp(0.3Al) (FIG.2D). The distinct feature observed in these samples was the presence ofβ-SiAlON, which increased as the amount of aluminum amount increased. Onthe contrary, α-SiAlON was present in relatively small amounts in sample3-Amp(0.1Al). Various aluminosilicates oxynitride phases were present inall amorphous samples. Contrary to what was observed in 3-α sampleseries, the N:O ratio decreased as more aluminum was incorporated.

Example 3 Microstructure of Formed Ca—SiAlON Ceramics

FESEM micrographs of samples 3-α, 3-α(0.1Al), 3-α(0.2Al) and 3-α(0.3Al)are shown in FIGS. 3-7. The grains in sample 3-α have two types ofmorphology: plate-like and semi-equiaxed structures. XRD analysis showedthat this sample consisted of β-SiAlON andSi_(3.1)Al_(2.9)O_(2.9)N_(5.1) phases. It is known that β-SiAlONexhibits an elongated morphology, which is apparent in the FESEM image(FIG. 3). The second phase Si_(3.1)Al_(2.9)O_(2.9)N₅₁ exhibitssemi-equiaxed grains structure, which was located within β-SiAlON plates(FIG. 4). As aluminum metal was incorporated in the reaction, the amountof α-SiAlON phase increased in samples 3-α(0.1Al), 3-α(0.2Al) and3-α(0.3Al), whereas the β-SiAlON phase disappeared in these samples,leaving several aluminosilicates oxynitride phases.

FIGS. 5 and 6 reveal that the aspect ratio of SiAl₅O₂N₅ phase is smallerthan that of SiAl₆O₂N₆ phase. Comparing these figures with FIG. 3, whichcorresponds to sample 3-α, the micrographs indicated that the level ofphase dispersion was enhanced as the amount of aluminum metal increased.

FESEM micrographs of 3-Amp sample series are shown in FIG. 8. The uniquefeature appearing in all samples is the dominance of β-SiAlON as a majorphase, and sample 3-Amp(0.3Al) had the most β-SiAlON. α-SiAlON formedsurprisingly in sample 3-Amp(0.1Al) (FIGS. 9 and 10). In addition tothese phases, different aluminosilicates oxynitride phases were present.It is prominent that β-SiAlON formed at the expense of these phases, asdemonstrated by samples 3-Amp(0.2Al) (FIG. 11), and 3-Amp(0.3Al) (FIG.12), in which the amount of SiAl₄O₂N₄ was reduced in sample 3-Amp(0.3Al)compared with sample 3-Amp(0.2Al). The observation was also confirmed byXRD results (FIG. 2).

Example 4 Effect of Adding Aluminum on Mechanical Properties of FormedCa—SiAlON Ceramics

Table 3 shows that replacing a portion of aluminum nitride by aluminumled to the enhancement of mechanical properties of the sintered samples.An increase of 27% in Vickers hardness and 13% in fracture toughnesswere achieved through 30% aluminum nitride substitution with aluminum.An enhancement of about 24% in fracture toughness was achieved by 20%aluminum nitride substitution with aluminum, i.e. sample 3-α(0.2Al).However, the hardness value decreased compared to sample 3-α (0.3Al).Aluminum may form aluminum nitride during the sintering under anatmosphere of nitrogen gas. Nitridation of aluminum usually takes placeat low temperatures, say around 530° C. and lower, but for prolongedduration, being different for different techniques and processingconditions (Visuttipitukul, Patama, Tatsuhiko Aizawa, and HideyukiKuwahara. Materials Transactions 44.12 (2003): 2695-2700, incorporatedherein by reference in its entirety).

TABLE 3 Experimental results of the samples sintered at 1600° C. SampleDensity (g/cm³) HV₁₀ (GPa) K_(IC) (MPa√m) 3-α 3.11 13.4 ± 0.7  7.2 ± 2.43-α(0.1Al) 3.15 13.7 ± 0.6  6.4 ± 1.5 3-α(0.2Al) 3.10  16 ± 0.5 9.5 ±1.9 3-α(0.3Al) 3.15 18.5 ± 0.7  8.3 ± 0.7 3-Amp 3.11 12.3 ± 0.8  5.2 ±1.3 3-Amp(0.1Al) 3.09 12.2 ± 0.3  6.1 ± 1.5 3-Amp(0.2Al) 3.10 13.3 ±0.4  5.3 ± 1.0 3-Amp(0.3Al) 3.09 14.4 ± 1.0  —

The presence of α-SiAlON is definitely responsible for the enhancementin the hardness values, as well as the increased fracture toughness dueto the crack deflection mechanism that is supported by themixed-morphology microstructure.

One possible mechanism for the increase in hardness could be that ahigher temperature is required for the α→β transformation (Mandal,Hasan. Journal of the European Ceramic Society 19.13 (1999): 2349-2357,incorporated herein by reference in its entirety). Another possiblemechanism would be the reverse transformation from β-SiAlON to α-SiAlONis favored under the aforementioned compositions of powder mixtures andsintering conditions.

Example 5 Formation of Elongated α-SiAlON Grains

It is interesting to note the formation of elongated α-SiAlON grains insample 3-α(0.3Al) (FIG. 13). Kurama and his group have reported thatelongated α-SiAlON grains may be formed if α-SiAlON formation ishindered at low temperatures by using fast heating rates duringsintering (Kurama, S., M. Herrmann, and H. Mandal. Journal of theEuropean Ceramic Society 22.1 (2002): 109-119, incorporated herein byreference in its entirety). Spark plasma sintering provides a fastheating rate, however, it should be noted that spark plasma sinteredsamples prepared by other research groups do not form elongated α-SiAlONgrains. Kurama has shown the necessity of having enough glassy phase toaid the elongation process that is thought to be more diffusioncontrolled from one side of the grain than the other (Li, Ya-Wen, et al.Materials Letters 47.4 (2001): 281-285, incorporated herein by referencein its entirety).

The presence of additional amount of the liquid phase is known to occurat high m and n values. Thus, it seems reasonable to assume that part ofaluminum formed extra liquid phase during the initial stages ofsintering.

Example 6 Role of Aluminum in Improving Sintering of Ca—SiAlON Ceramics

The 3-α series samples were sintered at a lower temperature, 1450° C.,for the same holding time (30 min). The corresponding micrographs aredisplayed in FIGS. 14-17 and the associated hardness and fracturetoughness values are listed in Table 4. It is apparent from these datathat the presence of aluminum metal improved the sintering of Ca—SiAlONceramics. The samples could be sintered at a lower temperature withminimal effects on their mechanical properties. To verify this, one maycompare sample 3-α sintered at 1600° C. and 3-α(0.3 Al) sintered at1450° C. (Table 4). The Vickers hardness increased from 13.4 GPa forsample 3-α to 19.6 GPa for 3-α(0.3 Al), while the fracture toughnessdecreased from 7.2 MPa√m for sample 3-α to 6.6 MPa√m for 3-α(0.3 Al).However, by comparing the sample 3-α sintered at both temperatures, onewould recognize a worse reduction in fracture toughness. Thus, addingaluminum metal precursor resulted in minimal negative effects on themechanical properties of Ca—SiAlON ceramics sintered at lowertemperatures.

TABLE 4 Summary of experimental results of the samples sintered at 1450°C. Density Sample (g/cm³) HV₁₀ (GPa) K_(IC) (MPa√m) Phases 3-α 3.17 17.0± 0.3 4.4 ± 0.8 α-SiAlON (S), SiAl₆O₂N₆ (W), SiO₂ (W) 3-α(0.1Al) 3.1618.6 ± 0.4 7.8 ± 0.8 α-SiAlON (VS), SiAl₅O₂N₅ (W), 3-α(0.2Al) 3.16 19.1± 0.2 6.4 ± 0.4 α-SiAlON (S), SiAl₅O₂N₅ (M), 3-α(0.3Al) 3.15 19.6 ± 0.36.5 ± 0.8 α-SiAlON (M), SiAl₅O₂N₅ (M), SiO₂ (W) VS: very strong, S:strong, M: medium, W: weak, VW: very weak

As a tentative rule, the aluminum amount is to be increased as thesintering temperature is raised up, to form α-SiAlON as a major phase.

Example 7 Effect of Aluminum Precursor on Amorphous Samples

In contrary to the positive role played by the aluminum precursor in the3-α series samples, the enhancement in mechanical properties ofamorphous samples was limited. The Vickers hardness started to increasewith 20% aluminum replacement, giving a maximum of 14.4 GPa for sample3-Amp(0.3Al). The presence of α-SiAlON in sample 3-Amp(0.1Al) did notlead to an increased hardness of the sample because α-SiAlON was presentin a small amount, as revealed by XRD and SEM results. The presence ofaluminosilicates oxynitride phases might decrease the hardness.Therefore, when 13-SiAlON phase grew at the expense of these phases, thehardness value was raised from 12.3 GPa to 14.4 GPa. The fracturetoughness values did not vary much because aluminosilicates oxynitridephases and β-SiAlON possess the same grain morphology but the respectivegrains have different aspect ratios, as demonstrated by FESEM and XRDresults. The formation of β-SiAlON in these samples was expected becauseamorphous Si₃N₄ would transform directly to β-SiAlON, which is the moststable phase of SiAlON.

The invention claimed is:
 1. A process for producing a Ca—SiAlONceramic, comprising: mixing calcium oxide; silicon nitride; alumina;aluminum nitride; and aluminum to form a powder mixture; sintering thepowder mixture to form the Ca—SiAlON ceramic; and cooling the Ca—SiAlONceramic to a temperature ranging from 20-40° C.
 2. The process of claim1, wherein the sintering is performed in an atmosphere consistingessentially of nitrogen gas.
 3. The process of claim 1, wherein themixing is at least one of sonication and ball milling.
 4. The process ofclaim 1, wherein the mixing proceeds for 10-30 minutes.
 5. The processof claim 1, wherein the calcium oxide is in the form of particles with adiameter ranging from 1-200 nm.
 6. The process of claim 1, wherein anamount of the silicon nitride ranges from 40 wt % to less than 80 wt %relative to a total weight of the powder mixture.
 7. The process ofclaim 1, wherein the silicon nitride is α-Si₃N₄ and in the form ofparticles with a diameter ranging from 1-200 nm.
 8. The process of claim1, wherein the silicon nitride is amorphous and in the form of particleswith a diameter ranging from 1-100 nm.
 9. The process of claim 1,wherein the alumina is in the form of particles with a diameter rangingfrom 1-200 nm.
 10. The process of claim 1, wherein the aluminum nitrideis in the form of particles with a diameter ranging from 1-200 nm. 11.The process of claim 1, wherein the aluminum is in the form of particleswith a diameter ranging from 10-100 μm.
 12. The process of claim 1,wherein an amount of the aluminum ranges from more than 0 wt % to 10 wt% relative to a total weight of the powder mixture.
 13. The process ofclaim 1, wherein the sintering is a spark plasma sintering process. 14.The process of claim 1, wherein the sintering is performed at atemperature ranging from 1400-1600° C.
 15. The process of claim 1,wherein the sintering comprises heating the powder mixture at a rateranging from 5-600° C./min.
 16. The process of claim 1, wherein thesintering comprises heating the powder mixture at a rate ranging from90-110° C./min.
 17. The process of claim 1, wherein the sinteringcomprises applying a uniaxial pressure ranging from 20-150 MPa to thepowder mixture.
 18. The process of claim 1, wherein the sinteringcomprises applying a uniaxial pressure ranging from 45-55 MPa to thepowder mixture.